To be clear, there may be questions about what dark matter is or isn’t, but gravitational lensing was the strong evidence that it exists.
The gravitational lensing also excluded the possibility that the missing mass was mostly comprised of MACHOs or objects like brown dwarfs etc… We know that the Standard model and GR are incomplete, and we have unanswered questions on what dark matter is or isn’t but it’s existence is well established.
A replacement theory for GR or the Standard Model wouldn’t simply disregard the current predictions and observations entirely, but must explain the observations that the existing theories account for. So a replacement theory that may be more complete will not simply discount the existing observations, but must actually account for them.
I add clarification that even dark energy, while not directly observed, has been demonstrated by using methods that reach the three sigma level, or about a 0.27% chance that the experiment is wrong.
Physics as stated above tends to prefer at least a five sigma level of confidence.
This article from nature made the news was saying that dark energy didn’t exist, but really it confirmed the previous tests, and said that dark energy is the model that best fits.
But as there is enough uncertainty that still results in that 0.27% chance that the experiment is due to those uncertainties it is termed “Marginal evidence”.
The existence of dark matter and energy will probably be qualified as unproven, despite the combined evidence moving past the five sigma level without direct observation.
Just keep in mind that most of the testing of these theories is typically limited by the precision of our testing devices, and consider that scale. As an example LIGO is measuring a change in arm length 1,000 times smaller than a width of a proton. For a century old prediction that is extraordinary.
The theory will be replaced at some point, and it doesn’t describe everything and thus is incomplete but consider the scale of changes a potential replacement could possibly have while meeting the need to describe the same observations.
From a field of physics perspective the changes of a new replacement theory may be massive and fundamental, but as a descriptive tool standpoint the Standard Model and GR are going to remain useful for the foreseeable future.
In fact in the case of the Standard Model, the bummer about the LHC is that it pretty much discovered what they expected it to discover and the frustration is that it hasn’t ruled out other replacement theories. While it is important to have direct observational evidence of the Higgs, it is a bummer that it was pretty much what was expected for several decades.
Dark matter has been detected, directly, using methods that have nothing whatsoever to do with gravity. For starters, we ourselves are technically dark matter: We have mass, and we don’t glow.
OK, but when people refer to dark matter, they mostly mean nonbaryonic dark matter. But we’ve detected that directly using nongravitational methods, too: Neutrinos are one component of nonbaryonic dark matter, and detecting them is routine nowadays (still very difficult, but routine).
Now, it must be admitted that neutrinos are only a very small component of nonbaryonic dark matter. But if there’s one uncharged fundamental particle out there which is very difficult to detect, why shouldn’t there be more? It’s a lot more unreasonable to suppose that all of the non-neutrino particles couple with electromagnetism, than it is to suppose that some do and some don’t.
It’s not just a trust in a model of gravitation. It’s a much deeper suite of measurements and physical phenomena all examined together, as the posted links (only partially) lay out. Sure, gravity is the dominant force in many of these phenomena, but something has to be. Gravity is the probe, not the heart of the discrepancies.
A blind person walks into a carpeted square room. She has been told the room is empty. She claps her hand and hears a faint early echo along with a louder late echo. She throws a handful of ball bearings into the room and hears dozens of thuds but also many metallic tinks. She hums (pitch perfectly) at increasing frequencies and notices that the room’s second harmonic is inexplicably louder than the first or third. She concludes that the room is not empty; that whatever’s in the room is reasonably large and consistent with being centrally located; that it’s surface is rigid.
All of her input came via sound, so we could say that she’s putting too much trust in a model of sound when drawing such conclusions. Or maybe there’s actually something in the room.
The link says that the galaxies should be flying apart based on the amount of matter holding them together. We see that gravity leasing also occurs in this area in the amounts we would expect to see to keep the galaxies together. The latter is not a confirmation of dark matter, but rather a restatement of the problem which is what is It that we can’t observe that is berating all this gravity?
The main hypothesis is that there is “dark matter.” Dark matter comes in two forms: first there is the space dust and rocks and brown dwarfs and all the other crap that we know is out there and can’t see. This makes up a small percentage of the dark matter that we must account for, and I don’t think anybody claims it’s not there. Second is this almost magical type of dark matter that nobody has ever seen or observed which only interact s with regular matter in weird ways on weird scales. In order for galaxies to hold together according to our observations this stuff needs to make up the majority of the mass in the universe. It is completely hiddden for us other than for the gravitational effects it seems to generate.
Now, the article points out exactly what I am saying. The idea that dark matter exists is one way to to model a solution for the problem, and even the best one at this time. Other groups of scientists are going a different route, which is to say that they are considering the possibility that we are missing something fundamental or have got something wrong in our models and they are working on a modified theory of gravity.
There may very well be dark matter. I don’t know. All i’m Doing is pointing out that we don’t have evidence of its existence. What he have is a problem. Dark matter is the hypothesis that potentially solves that problem within our model. What we need now is evidence to confirm the hypothesis. We don’t have that yet.
If you are going to imply that we are all cold-blooded bastards here, I will report you. I am pretty sure that you do glow, that is radiate heat. I’m also pretty sure you show up on infrared.
I’m glad you presented that fairly. The big problem is that if our understanding of physics and gravitation is correct the majority of the universe must be made up of such a hypothetical particle.
I’m not saying it’s not there. I sure as hell don’t know. Nobody does, because we haven’t found any evidence for it. What we have are guesses ranging from the educated to the pretty wild ass ones. Another possibility, no better, no worse is that we are either wrong about something fundamentally, or are completely missing something. I find this exciting. If we are missing something or wrong it might be that it is something beyond our limits of understanding and we’ll never get it, but it might also mean that when we figure out it will open up exciting new areas to science, and maybe even have applications her on earth.
You’ve conflated two observations and ignored many others. I expand below.
Why is it “almost magical”? From a particle physics perspective, the required properties of dark matter particle(s) are quite sensible and not weird at all.
Again, galaxy rotations are only one of many pieces of evidence for dark matter.
It is only hidden from a practical point of view. But either way, why is that a problem? 130 years ago all subatomic particles were completely unknown to us. In only the last half-century have we been able to detect a neutrino, and it was only 18 years ago that the tau neutrino specifically was observed. The Higgs particle was only detected in 2012, and only after investing some $40,000,000,000 over four decades (a rough sum across Tevatron, LEP, and LHC attempts to find it).
Nature doesn’t owe us simplicity of discovery. Why shouldn’t there be a new particle that’s hard to find. Plenty of other ones have been hard.
In cmyk’s linked article, the author tries to be Fair and Balanced by discussing the modified gravity possibility at some length. But she buries the lead in her closing, noting that “So far, these theories … have not yet provided an explanation for the complete set of observations like dark matter [has].”
And that’s the nicest way she could have said the truth, that “this idea has been dead for a long time.” See, back when galaxy rotation curves were first measured, they were the only glimpse of something new happening. At that time, it made complete sense to consider all possible ways you could explain that one peculiarity. But as independent clues trickled in over the years, you quickly had no more ways of getting around the fact that you need actual stuff in actual space to explain all observations. Some of the initial proponents of modified gravity have clung to their falsified pet theories with an icy grip to their detriment. Science requires letting go and moving on when presented with new information.
We have ample evidence. I outline some of it here. Feel free to ask for clarifications. There are a lot of independent pieces, so I jump from one to the next quickly in this first pass.
Galaxy rotation curves – how quickly objects revolve about the core of galaxies – can’t be explained by standard gravity given the amount of visible mass. Some amount of additional mass, distributed in just the right way, would fix this.
Galaxy cluster behavior is inconsistent with the visible sources of gravitational potential energy. Take care not to conflate these first two items. Stars moving in a galaxy is a very different scenario to thousands of galaxies influencing each other from afar. In this latter case, each galaxy is it’s own billiard ball influencing and being influenced by the other billiard balls, and you can infer the masses of each.
Already you have problems if you try to fix star speed by modifying gravity just so. You need just the right finely tuned changes to even make that work, yet you now need to also explain galaxy clusters, which are phenomena occurring on distance and time scales thousands of times different from those within a galaxy.
However, if you posit the presence of a new particle that interacts only very weakly, then the amount you need to make stars move correctly and the amount you need in each galaxy to make entire galaxies move correctly (on very different spatial and temporal scales) matches right up.
Galaxy cluster formation over cosmological time scales. Clusters far away are at a younger stage of development than ones nearer by. One can use this fact to see how clusters form and evolve over time. This time evolution suggests the presence of dark matter in an amount consistent with other evidence.
The Bullet Cluster collision observations (and the gravitational lensing induced by the cluster) reveal distinct types of material present in the clusters, as stars, gases, and dark matter would move differently through the collision process. There have since been many other cluster collisions measured, and all point to the same required amount of dark matter. To be clear, there is incontrovertible evidence that there are two distinct components of mass in the cluster, one of which is approximately interaction-less and one of which acts like normal matter. These components end up with displaced spatial distributions in collided clusters, in stark contrast to non-collided clusters where a displacement of the two components is, naturally, not seen. The amount of dark matter inferred is consistent with other dark matter evidence.
Lensing, both strong and weak, can map out matter density profiles from the level of galactic substructure (“short” distances) up through clusters and superclusters. These lensing measurements reveal the presence of unseen matter in consistent amounts.
Measurements of the details of large scale structure match the predictions of our cosmological model very well, but only if some non-relativistic, weakly interacting matter component is present during early structure formation. And the amount needed matches the rest of the dark matter evidence.
The cosmic microwave background (CMB) radiation provides information on the matter and energy present very early in the universe. The most famous related plot is the anisotropy power spectrum, which shows how much point-to-point correlation there is in the CMB sky map at different angular scales (e.g., how much correlation there is between points right next to each other versus between points on opposite sides of the sky, say). These features are echoes of the random density fluctuations present in the freshly born universe. However, those density fluctuations come in two varieties for our purposes here: dark matter (approximately non-interacting) and baryons (interacting). High density regions tend to grow gravitationally, but baryonic matter interacts, heats up, and resists clumping. You even get density “bouncing” – oscillations in the baryonic component of the matter. Dark matter doesn’t interact this way and thus has a very different influence on the evolution of density fluctuations (which, by they way, turn into galaxies and larger structures much later on. There are also photons and neutrinos that enter the story, but they’re not critical for this post.) The amount of – and properties of – dark matter inferred from the CMB again matches the other evidence. This isn’t a subtle effect, either. Without dark matter, the exquisite agreement in the above-linked power spectrum is a goner.
Many pieces of evidence require dark matter to be non-baryonic, with one important item being the relative abundances of light nuclei in the universe. You can’t have a ton of extra baryonic matter hanging around in the early universe without completely wrecking the relative abundances of, say, deuterium and hydrogen-1.
The cosmological models that yield successful predictions of these last few items make other predictions that are also well tested using other independent observation channels (supernovae, Lyman-alpha forest). The point here is that one can’t try to dodge the cosmological-scale evidence for dark matter with model changes (not that there’s a need to) since those changes would have dire consequences on other observations not itemized here.
As warned, that’s a lot to read through, but I encourage you to do so if you are interested in the topic, and to ask follow-up questions if you’d like. In particular, a cursory skim may give you the incorrect impression that the same thing shows up multiple times. In fact, these are (with minor exceptions) rather independent probes despite words like “galaxy” and “gravity” showing up in multiple places. (See: “sound” analogy in my earlier post.)
Bottom line: there is simply no way to accommodate all this independent and consistent evidence aside from the rather elegant solution we already have: there is actual stuff in actual space with a fairly constrained set of possible properties.
Pasta, will you indulge me and let me re-ask one of my favorite questions?
How do “we” know that dark matter does not interact with photons?Chronos has answered this question for me at least twice before — and his answers seem better than the answers at Physics.Stackexchange, but I didn’t fully understand them.
It seems to me that ordinary stars emit a lot more light than they absorb. Surely, at least in a thought-experiment à la the demons of Maxwell and Boltzman, we could imagine an object which absorbs a lot more light than it emits.
In the short term, sure. Start out with an object which is, for some reason, colder than its surroundings, and it’ll absorb more than it emits until its temperature comes up. But it’s tough to get something colder than its surroundings to begin with, and in most cases it would take a lot less than the lifetime of the Universe for it to come up to temperature.
That’s a great and informative post. You put a lot of work into it. Thank you.
In its repetition of previous content it indicates that you may not have understood what I was saying. In that case I will take the blame for my poor explanatory skills. I’ll try again:
On the cosmological scale there is about 5 times as much gravity being generated as we would expect based on the amount of matter we can account for.
So far, the best hypothesis for this is dark matter, which stated simplistically is the idea that for every pound of regular old stuff that makes up the universe of beer and fish tanks that we live in, there is actually five pounds of invisible stuff that doesn’t interact with the universe of beer and fish tanks (ubft from here on out.,). We can’t see it or detect it yet. For our purposes we’ll ignore the WIMPs and MACHOs that would be a Ubft explanation for dark matter for now. This invisible stuff is 5 times as common as ubft stuff so it is probably all around us. If we accept this, than suddenly everything makes sense again from a cosmological gravitational standpoint (never mind about dark energy.)
So, now we have to find evidence of this stuff, right? I say there is not any, you say there is tons of it.
The reason why I disagree with you is that I believe your logic is circular. Let’s say I buy a 100 pound bag of rice. When I take it home and weigh it it weighs 500 pounds. How could this be? I propose that there is an invisible dragon who weighs 400 pounds sitting on the bag of rice. If you try to touch him your hand passes right through him, because, duh, he’s a magic dragon. Or, if magic offends you we can say he is out of phase with our reality yet nonetheless exerts a weighty presence. If I accept the premise of this dragon I have now solved the problem of why my 100 pound bag of rice weighs 500 pounds. I am proud of myself, I tell you the story of how I solved the mystery.
You, being a gentleman, and perhaps being wary around lunatics, congratulate me on producing a hypothesis that explains the discrepancy. You then ask what is my evidence for the invisible, untouchable, massive dragon. I tell you that there is 400 pounds of extra weight to be accounted for so clearly some kind of massive dragon must be on that bag of rice. I also tell you that I put the bag on a sled and pushed it around, and confirmed that it is five times harder to push around than a 100 pound bag of rice should be. I also used a pair of binoculars and confirmed that when I looked near the rice things past the rice were magnified more than they should have been. In fact they were magnified exactly as much as they should have been as if my dragon was sitting on it. I show you other experiments which really just confirm that the bag weighs more than it should.
You say yes, the bag weighs more than it should. The dragon would explain why it does, but the fact the bag weighs more than it should dis not evidence for the existence of your dragon. You are using circular logic.
Similarly, all of your evidence for dark matter is just confirmation of the original problem which is that we are observing the effects of 5x the amount of gravity than we would expect from the stuff we can see.
Now perhaps you think my explanation using dragons is unfairly fantastical? In fact though is most exactly what is being proposed by some. In the July/August 2013 issue of Discover magazine discusses Randall and Fans theory of Matter which basically suggests that there is a whole parallel galaxy of dark matter on top of our own, and this shadow galaxy might even have shadow stars, planets, and… god forbid, invisible dragons. The article is called “The possible parallel universe of dark matter.”
So, I have nothing against dark matter. But, we are still in the guessing stage of what it is and ll ides are clearly still on the table. What you are claiming as evidence is really just a restatement of the problem, and the problem begging the solution is not evidence supporting it.
Because you know, maybe I just accidentally left the bag under a water leak, and it soaked up 400 pounds of water. Wouldn’t I be embarrassed about proposing dragons if that were the case?
One big difference: You’re saying “400 pounds of dragon”, rather than “400 pounds of something”. What makes you say the something is a dragon?
And you might object that the problem might be with your scale, instead of with the rice. Which it might be, if you only use one scale. But what if you weigh it on seven different scales, and find the same weight each time? Are you going to object “Well, those aren’t actually separate measurements, because they’re all scales”?
I called it. Dragon because it ighlights the fanciful properties of being invisible and untouchable And described without ever having been observed… like dark matter. Plus, I just like dragons.
In my example, I know the scale is right because of the pushing around the rice on the sled, and the lending with the binoculars.
The problem with dark matter as an explanation is that it basically says “there is some magic stuff, that explains this problem”. Same with dark energy. These are just placeholders we’ve given to stuff we haven’t figured out yet. With dark matter we think we are shooting blind down a pretty narrow corridor. We have a pretty good idea that we are going to hit somebody. With dark energy we are shooting blindly into the woods.
Dark energy pretty much is just a placeholder name: We know that there’s this phenomenon, and we know that there must be something causing it, but we have less than no clue of what it is.
Dark matter, though, is much firmer: We still don’t know what it is, but we know what sort of thing it can be, and we know a lot of its properties.
Nope. We don’t. It could be more than one thing, or it could be something that is emergent in other things we already know about but which have a property or properties we don’t know about, or it could be tied in with dark energy and we are really only looking for one thing, or… you get the idea.
All we actually think we know is that there seems to be a lot more mass than we can account for and a couple of clues are suggestive that maybe it’s a real form of matter that has mass but is conventionally undetectable. Something which has a lot of mass and is conventionally undetectable sounds a lot like our imaginary dragon, which is to say that we really don’t know much about it at all.
One of the big no nos of science is getting attached to an unproven hypothesis. If you spend all your time looking for the thing you think you know, you can miss the thing you are actually looking for. The danger isn’t what you don’t know, it’s what you think you know. What we have are clues that are suggestive.
Our observations come from a tiny sliver of time from the universe’s standpoint, and they have been made from a paltry assortment of instruments from our own narrow human perspective. It is hard to overstate how small a sample we have managed to get from which to extrapolate our understanding of the universe. It is a testament to our ingenuity and persistance that we have done so much with it. But, our window into the universe is the tiniest of slivers. Stagnation comes when we get arrogant in our beliefs.
Theories of dark matter run from baryonic non radiative things, MACHOS, to WIMPS, to shadow galaxies, to changes in the gravity model (I forget the acronym) and a bunch more. Some of these are mutually exclusive.
You cant be that up in the air and think you know a lot about something. We only look good on dark matter because we are so absolutely clueless on dark energy.
I have only a minute to check in here. More later. For now, though…
I agree with your assessment of your rice example, but it is not a good analogy. It would have been a good analogy to the dark matter situation back when we only had galaxy rotation curves.
I believe the difficulty may be in recognizing how radically independent the different probes of dark matter are. Yes, most involve the word “gravity”. But the rice example would be a much more correct analogy if you noticed the bag weighed too much on the scale and it also came back as having too much mass when measured with a centripetal scale and it also came back as having too many particles present when its behavior is examined under extreme temperature and pressure and the rice-on-rice interactions behave as if they are influenced by the presence of other particles when squished in a bag or when dispersed in a wind tunnel. If all these point to the same story – the presence of a different type of particulate matter in the rice with a consistent concentration – you can no longer say it’s not there.
I recognize that the differences in the various dark matter evidences may not be immediately obvious without getting into the technical details, but that’s where I think follow-up would be most fruitful.
Perhaps revisit the “sound” analogy up-thread as well, which is similar in spirit to your rice example but has a key feature in its favor: it uses a single sense (sound) to make multiple independent measurements, akin to how many of the dark matter pieces of evidence happen to use gravity as the sensor, but in very different ways. (This is key!)
Would it be fair to summarize as follows? :—
*The main reason physicists know dark matter does not absorb photons is due to arguments based on the 2nd Law of Thermodynamics.
*
Please note that I am NOT asking for any explanation of WHY the 2nd LoT leads to that conclusion, NOR do I hope to debate the 2nd Law :smack: NOR do I wish to hijack the thread into any discussion of mythological creatures (unicorns, Loch Ness ‘Nellie’, Maxwell’s demon). I only want to know if many physicists would agree with the indented sentence above.
Some of the “answers” at Physics.Stackexchange seem to depend on that same 2nd LoT connection but neither there nor at laymen’s sources like Wikipedia is the connection over stated explicitly.
